Conductive cement formulation and application for use in wells

ABSTRACT

The invention provides a cement slurry composition for cementing a well comprising a hydraulic cement, water, carbon fiber and graphite. Compositions of the current invention combine the benefits obtained from adding carbon fiber and graphite to the cement composite. The synergy achieved from combining fibers and particulates into the same sample results in a composite slurry with improved electrical properties and easy-to-optimize rheologies.

FIELD OF THE INVENTION

The present invention broadly relates to well cementing. Moreparticularly the invention relates to a conductive cementing compositionand methods of placing such cementing composition in a well, such as anoil or gas well.

DESCRIPTION OF THE PRIOR ART

Cement in oil and gas wells is typically placed in the annular gapbetween the drilled formation and the steel casing. Its primary functionis to prevent any fluid communication between the drilled formations inorder to provide long-term zonal isolation. Zonal isolation must beachieved during the life of the well and after its abandonment. Cementhas been used for more than seventeen years in oilwell applications.Also, cement has great versatility as an engineering material,demonstrating superior compressive and tensile strengths, ductility, andflexibility over a wide density range, depending on the additives chosenduring design.

However, conventional cements also typically exhibit high electricalresistivity values and are generally considered good insulators ofelectrical current. This property can be an advantage or a disadvantagefor certain applications. For example, in one study, the measured dryresistivity values of cement range from 6.54×10³ to 11.4×10⁵ Ω percentimeter. Other studies have reported that the addition ofparticulates and fibrous conductive materials may significantly improvethe electrical properties of the cement composite materials. With theproper addition of conductive materials, acceptable cement electricalproperties have been achieved with standard Portland cement or concretecomposites. Most of these studies have been performed on standard Type Ior other construction-grade cement.

Oilwell cement, on the other hand, is exposed to a different set oftemperature and pressure conditions, depending on depth and lithology.Moisture conditions also change from well to well. Since cement is aporous material, and porous mediums have been found to follow Arps lawwith respect to temperature, it is important to include cement porosityas a variable in resistivity measurements. Two types of porosity shouldbe considered in this discussion: initial and final porosity. In initialor “slurry” porosity, the ratio of mix water to cement slurry isexpressed as a percentage of total volume. The final or “set” porosityis expressed as the ratio of pore/void volume to total volume of the setmaterial. Based on earlier findings, resistivity of set cement isdirectly proportional to the final connected porosity, as well as theionic character of the interstitial fluid. Final results suggested thatextended cement samples with high final or “connected” porosity (˜59.5%)had the best conductivity of the samples tested.

The major drawback to high-porosity cement systems is the dramaticreduction in mechanical and long-term zonal isolation properties, whencompared to lower-porosity cement samples. Therefore, extended systemshave not been considered a suitable long term solution for cased holeformation resistivity measurements.

For this reason, it is important to develop a low-porosity oilwellcement technology with excellent electrical properties and independentof initial or final porosity.

SUMMARY OF THE INVENTION

The invention provides a cement slurry composition for cementing a wellcomprising: a hydraulic cement, water, carbon fiber and graphite.Compositions of the current invention combine the benefits obtained fromadding carbon fiber and graphite to the cement composite. The synergyachieved from combining fibers and particulates into the same sampleresults in a composite slurry with improved electrical properties andeasy-to-optimize rheologies.

Preferably, the carbon fiber is present in an amount not exceeding 5 kgper cubic meter and more preferably, between 0.5 and 2 kg per cubicmeter.

Preferably, the graphite is present as coarse particulate graphite in anamount not exceeding 50% by weight of dry blend and more preferably, inan amount between 20% and 50% by weight of dry blend.

In another embodiment the slurry further comprises carbon blackconductive filler not exceeding 1% by weight of dry blend.

In another aspect, the invention provides a dry cement blend comprisinga hydraulic cement, carbon fiber and graphite.

In another aspect of the invention, a method of cementing a well isprovided wherein the method comprises the step of pumping a slurrycement composition comprising: a hydraulic cement, water, carbon fiberand graphite. Advantageously, the carbon fiber is present in an amountnot exceeding 5 kg per cubic meter and more advantageously, between 0.5and 2 kg per cubic meter. Also advantageously, the graphite is presentas coarse particulate graphite in an amount not exceeding 50% by weightof dry blend and more advantageously, in an amount between 20% and 50%by weight of dry blend.

Preferably, the method comprises the step of drilling the well andputting in a casing, wherein the step of cementing applies the cementaround the casing. And the method further comprises the step ofdeploying a tool able to measure formation resistivity through casingand measuring said formation resistivity.

The new formulation of the invention will yield a cement sheath having aresistivity that will be at least 1-2 orders of magnitude below theformation resistivity. This will allow more signal to pass through thecement into the formation and improve penetration radius, as well asincrease accuracy and resolution in the measurements of resistivity.While any available resistivity measurement tool may be used to measurein wells having cement compositions of the invention, preferably,measurement of the formation resistivity is done with a Cased HoleFormation Resistivity Tool (CHFR) provided by Schlumberger.

In another aspect, a method of measurement is disclosed wherein themethod measures the formation resistivity of a well wherein the wellcomprises cement comprising carbon fiber and graphite.

Still in another aspect of the invention, the slurry of the inventioncan be used as a cathodic protection for a well.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments of the present invention can be understood with theappended drawings:

FIG. 1 shows the experimental setup for resistivity measurements.

FIG. 2 shows the impact of carbon black on cement resistivity.

FIG. 3 shows the impact of carbon fiber on cement electrical properties.

FIG. 4 shows data demonstrating significant improvement in conductivitywhen adding carbon fibers to cement composite with carbon black andcoarse graphite.

FIG. 5 shows relationship between fiber connectivity and cementcomposite electrical properties.

FIG. 6 shows influence of coarse particulate graphite on cementresistivity.

FIG. 7 shows impact of coarse particulate graphite on cementconductivity and the synergy between coarse particulate graphite andfibrous graphite.

FIG. 8 shows impact of shear on carbon fiber integrity and electricalproperties of cement composite samples.

DETAILED DESCRIPTION

At the outset, it should be noted that in the development of any suchactual embodiment, numerous implementation—specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem related and business related constraints, which will vary fromone implementation to another. Moreover, it will be appreciated thatsuch a development effort might be complex and time consuming but wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure. The description and examplesare presented solely for the purpose of illustrating the preferredembodiments of the invention and should not be construed as a limitationto the scope and applicability of the invention. While the compositionsof the present invention are described herein as comprising certainmaterials, it should be understood that the composition could optionallycomprise two or more chemically different materials. In addition, thecomposition can also comprise some components other than the onesalready cited. In the summary of the invention and this detaileddescription, each numerical value should be read once as modified by theterm “about” (unless already expressly so modified), and then read againas not so modified unless otherwise indicated in context. Also, in thesummary of the invention and this detailed description, it should beunderstood that a concentration range listed or described as beinguseful, suitable, or the like, is intended that any and everyconcentration within the range, including the end points, is to beconsidered as having been stated. For example, “a range of from 1 to 10”is to be read as indicating each and every possible number along thecontinuum between about 1 and about 10. Thus, even if specific datapoints within the range, or even no data points within the range, areexplicitly identified or refer to only a few specific, it is to beunderstood that inventors appreciate and understand that any and alldata points within the range are to be considered to have beenspecified, and that inventors possession of the entire range and allpoints within the range.

According to the invention, the slurry cement composition for cementinga well comprises an hydraulic cement, water, carbon fiber and graphite.The most commonly reported carbon fiber for improving electricalproperties in set cement has an average particle size equal to orgreater than 6 mm. Similar electrical properties were achieved with 3and 6 mm fibers but the addition of 3 mm fibers rendered the cementslurries unmixable. The slurries developed unacceptable Theologicalproperties. For this reason, longer fibers are more useful in oilwellcement applications. Graphite is used as coarse particulate graphiteaverage diameter is around 70 to 500 μm for the particle size.

Portland cement containing carbon fiber and particulate graphitedemonstrates reduced cement resistivity values, when compared to theresistivity values of conventional cement with no fibers or graphitepresent. Small concentrations of carbon fiber provide a connective paththrough the cement matrix for electrons to flow.

Higher fiber concentrations result in slightly improved conductivity,but adversely affect cement slurry mixability/pumpability. Althoughhigher particulate concentrations of graphite are required to improveconductivity, mixability remains acceptable. Therefore current inventionis designed to significantly improve conductivity of conventional cementformulations, independent of density and porosity of the cementcomposite. Cement composites with varying porosity (46-49%) and density(15.2-16.4 ppg) have been tested with predetermined concentrations ofcarbon fiber and coarse particulate graphite. Preferably, carbon fiberis present in an amount not exceeding 2 or 5 kg/m³ and the coarseparticulate graphite is present in an amount not exceeding 50% BWOB (byweight of dry blend).

Other additives may be present in the blend such as filers, retarders,fluid loss prevention agents, dispersants, rheology modifiers and thelike. In one embodiment, the blend also includes a polyvinyl chloridefluid loss additive (0.2-0.3% BWOB), polysulfonate dispersant (0.5-1.5%BWOB), carbon black conductive filler aid not exceeding 1.0% BWOB, andvarious retarders (lignosulfonate, short-chain purified sugars withterminal carboxylate groups, and other proprietary synthetic retarderadditives). In some formulations, silica or other weighting additives,such as Hematite or Barite, may be used to optimize rheologicalproperties of the cement composite slurry during placement across thezone of interest. Typically silica concentrations will not exceed 40%BWOC (by weight of cement). This is done to prevent strengthretrogression when well temperatures may exceed 230° F. For mostformulations, Hematite or Barite does not exceed 25% BWOB or BWOC.

While the addition of either carbon fiber (˜2-5 kg/m³) or particulategraphite (40-50% BWOB) signally provides some improvement to cementcomposite conductivity, evidence shows that composites with bothcomponents act synergistically and so the combination providesunexpected improvement in promoting conductivity as it will be show inthe following examples.

Experimental Set-Up

There are two types of resistivity measurements, electronic andelectrolytic, that characterize conductivity in oilwell cements. Thefirst type of measurement is due to the movement of electrons throughthe conductive phase (i.e. carbon fibers and particulate graphite) andthe second type of measurement is due to the motion of ions (i.e. K²⁺,Ca²⁺, Na⁺) in the pore space. Since cement placed downhole is constantlyexposed to various brines and moisture cannot easily escape the cementmatrix, these two types of conductivity cannot be easily decoupled.Therefore, in this study, each sample was constantly exposed to ahumidified environment at simulated downhole temperature (150° F.). Atperiodic intervals over a 30-day testing period, each sample was removedfrom the bath. Excess moisture was wiped off and the cells were allowedto equilibrate with ambient temperature for two hours. The electrodeswere connected to the RCL Meter for measurement (as shown on FIG. 1). Analternative current signal was used because the specimens containedmoisture and polarization effects were possible at specimen-electrodeinterfaces. The raw data were collected in resistance units andresistivity was calculated with the following expression:

$\begin{matrix}{\rho = {\frac{1}{\sigma} = {\frac{S}{L} \cdot R}}} & (1)\end{matrix}$where ρ is the resistivity, σ is conductivity, S is the cross-sectionalarea of the conductive path, L is the path length, and R is theresistance.

The properties of resistivity and conductivity are inverselyproportional. A testing matrix was chosen to consider the synergybetween two different sized particulates (carbon black and coursegraphite) and two different sized graphite fibers (3 and 6 mm). Thematrix is provided in Table 1 below.

TABLE 1 Composite Blend Components Physical Properties Carbon CarbonCourse Black Fiber Graphite Density Resistivity Sys- (% (gms/ (% g/cm³(ohm · meter) tems BWOB) mL) BWOB) SVF (ppg) 5 days 30 days 1-1  0 0.0000 47.6 1.99 0.081 0.141 1-2  0.001 (16.6) 0.032 0.047 1-3  0.002 0.0240.041 1-4  0.003 NA NA 1-5  0.010 0.006 0.010 1-6  0.000 50 47.8 1.840.039 0.084 1-7  0.001 (15.4) 0.013 0.038 (6 mm) 1-8  0.002 0.006 0.020(6 mm) 1-9  0.001 0.016 0.041 (3 mm) 1-10 0.002 0.008 0.019 (3 mm) 1-111 0.000 0 47.0 1.96 0.086 0.102 1-12 0.001 (16.4) 0.035 0.048 1-13 0.0020.017 0.025 1-14 0.003 0.011 0.021 1-15 0.000 20 46.5 1.87 0.073 0.1451-16 0.002 (15.6) 0.015 0.055 1-17 0.003 0.018 0.042 1-18 0.005 0.0060.032 1-19 0.000 40 46.9 1.82 0.051 0.064 1-20 0.001 (15.2) 0.023 0.036

Table 1 is a summary of the different systems tested to develop cementcomposites with superior electrical properties, from acceptablemixability and pumpability in the field to develop superior electricalproperties in the set cement.

The cement used in this study was Portland API Class G. The measuredcomposition of Class G is 55 wt % C₃S, 22-28 wt % C₄AF, 5.0 wt % C₃A,2.9% SO₃, 0.8% MgO, 0.55 wt % Alkalies (Na₂O·0.66 K₂O), and other tracecomponents. For the purposes of discussion, the following nomenclatureis used to describe systems tested in this study: Example: 0-0.002-0 (0%BWOB carbon black-0.002 gms/mL carbon fiber-0% BWOB coarse graphite).I.e., the first number represents the concentration of carbon black in %BWOB, the second number represents the concentration of carbon fiber ingms/mL, and the last number represents the concentration of courseparticulate graphite in % BWOB.

EXAMPLES Example 1

The relationship between carbon black and cement resistivity issummarized in FIG. 2. Two different studies were performed to determinethe influence of carbon black on cement resistivity. In both studies,carbon black had little or no effect on cement resistivity.

Example 2

The next study focused on the relationship between carbon fiberconcentration and cement conductivity. FIGS. 3 and 4 summarize theresults. In FIG. 3, the carbon fiber was varied from 0 to 17 kg/m³. InFIG. 3, the effect of carbon fiber (6 mm) on cement resistivity wasmeasured with carbon black held constant. Beginning with the addition of2 kg/m³ carbon fiber, the cement resistivity was considerably reduced.After 30 days curing at 150° F., cement resistivity with 5 kg/m³ was0.043 Ω.m. Further improvement was observed at 10 kg/m³ but with asubstantial increase in viscosity this concentration was deemedunsuitable for oilwell cementing applications. In FIG. 4, an additionalstudy evaluated the influence of carbon fiber on cement resistivity witha cement sample containing coarse particulate graphite (40% BWOB).Again, the addition of carbon fiber significantly reduced cementresistivity from 0.065 to 0.037 Ω.m after 30 days curing at 150° F.Testing with higher fiber concentrations was not feasible due toviscosification of the sample.

During testing of carbon fiber systems, a threshold or percolationeffect was observed. According to one study, fibers at lowconcentrations collect in packets with high localized conductivity butlow connectivity across the cement matrix. At a “threshold” fiberconcentration, the fibers have enough interconnectivity across thecement matrix to conduct a current. FIG. 5 demonstrates the effect ofincreasing fiber concentration on cement resistivity after 10 dayscuring at 150° F. In this study with 1% BWOB carbon black, carbon fiberdemonstrated a threshold between 5-9 kg/m³. An increase to 9 kg/m³carbon fiber failed to significantly improve the electrical propertiesof the sample. An illustration of the threshold concept is placed besideeach measurement. This demonstrates the link between increased fiberconnectivity and improved electrical properties through the sample.

Example 3

The effect of coarse graphite particles on cement resistivity is plottedin FIGS. 7 and 8. In FIG. 7, the measurements were performed under twoconditions: no carbon fiber and carbon fiber at a concentration of 5kg/m³. Coarse particulate graphite had a significant effect on cementresistivity. After 30 days curing at 150° F., 20% BWOB graphite hadlittle or no effect on resistivity. However, between 20-40% BWOB, thereappeared to be enough proximity between adjacent graphite particles inthe cement matrix to improve the electrical properties.

The relationship between particulate graphite concentration and cementresistivity are demonstrated in FIG. 8. In this study, the impact ofcoarse graphite was first studied without carbon fiber present in thecomposite. The carbon fiber was added at the threshold value (2 kg/m³).In the sample without carbon fiber, the addition of particulate graphitehad a significant effect on resistivity. There appears to be a“threshold” effect . . . similar to carbon fiber cement compositesamples. In the samples containing carbon fiber and particulategraphite, the effect of particulate graphite on cement electricalproperties was not as significant but still measurable. The resistivitywas reduced from 0.180 to 0.007 Ω.m when particulate graphiteconcentration was increased from 0 to 50% BWOB, respectively. Frommeasurements obtained in this study, there appeared to be a synergisticrelationship between coarse particulate graphite and carbon fibermaterials. However, in samples tested before, addition of coarseparticulate did not appear to lower the threshold for cement compositescontaining carbon fiber. The threshold for carbon fiber compositesremained between 2-5 kg/m³.

Example 4

In order to determine the effect of shear on carbon fiber integrity andthreshold required to express acceptable electrical properties, a cementcomposite sample with 17 kg/m³ carbon fiber was exposed to two differentshear environments and compared to a control sample. From thispreliminary study, high shear rates have a significant and irreversibleeffect on cement conductivity. After 1 minute shearing at 12,000 rpm,carbon-fibers were degraded to very small particles and lost theirfibrous character. Carbon fiber cement samples lose their electricalproperties after exposure to high shear rates for even short periods oftime, with resistivities very similar to conventional cement.

Finally, all particulate conductive materials were added to the cementpowder prior to mixing as part of the blend and were sheared at 12,000rpm. Further, all measurements of coarse graphite particles showexcellent conductivity at higher concentrations (greater than 40% BWOB).Therefore, it is essential that for a high-shear environment (12,000rpm) that might be faced when cement composite slurries are circulatedthrough centrifugal pumps during field placement, particulate graphitebe used in combination with carbon fiber to offset potential shearingdamage to the fibers. Since fibers provide better overall conductivityand particulate graphite provides better shear resistance, it isrecommended to use both materials in cement composites used in oilwellapplications where high shear environments are anticipated.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails herein shown, other than as described in the claims below. It istherefore evident that the particular embodiments disclosed above may bealtered or modified and all such variations are considered within thescope and spirit of the invention. Accordingly, the protection soughtherein is as set forth in the claims below.

1. A cement slurry composition for cementing a well comprising ahydraulic cement, water, carbon fiber and coarse particulate graphitehaving an average particle size of from about 70 to about 500 μm.
 2. Theslurry of claim 1, wherein the carbon fiber is present in an amount notexceeding 5 kg per cubic meter.
 3. The slurry of claim 2, wherein thecarbon fiber is present in an amount between 0.5 and 2 kg per cubicmeter.
 4. The slurry of claim 1 wherein the slurry is formed by mixing adry blend comprising hydraulic cement, carbon fiber and graphite withwater.
 5. The slurry of claim 4, wherein the graphite is present in thedry blend in an amount not exceeding 50% by weight of the dry blend. 6.The slurry of claim 4, wherein the graphite is present in an amountbetween 20% and 50% by weight of the dry blend.
 7. The slurry of claim4, further comprising carbon black conductive filler not exceeding 1% byweight of the dry blend.
 8. A dry cement composition comprisinghydraulic cement, carbon fiber and coarse particulate graphite having anaverage particle size of from about 70 to about 500 μm.
 9. Thecomposition of claim 8, wherein the graphite is present in the dry in anamount not exceeding 50% by weight of the dry blend.
 10. The compositionof claim 8, wherein the graphite is present in an amount between 20% and50% by weight of the dry blend.
 11. The composition of claim 8, furthercomprising carbon black conductive filler not exceeding 1% by weight ofthe dry blend.